The present invention relates generally to paper making waste sludge processing, and more particularly, to methods for dry recovering fibers from paper making waste sludge and to the fiber products and filter fractions made according to the methods.
The manufacture of paper involves blending a pulp material (generally wood fiber) with fillers, such as clay, and other additives in water to create a stock slurry mixture. The slurry is then processed through a paper making machine to form a sheet. The water is then extracted from the sheet which is then pressed and dried thereby forming paper product. During the paper making process the water is drained. This water contains an amount of fibers and filler material. It is known to collect this material for later processing, however, the recovery is usually not complete. In other instances batches of slurry may not be used and are discarded. The discarded material and material not captured for reuse is generally transported to a waste treatment facility located near the paper making mill where the solids, i.e., the fibers and filler materials, are removed from the water and slurry and the cleaned water discharged back into the environment or communicated back to the paper making process for reuse. After such dewatering the solids are contained in a concentrated, typically 40%-60% solids, slurry or sludge. The main components of this sludge are fibers and clay filler material. This sludge is usually disposed of by burying in landfills, landspreading, or incinerating.
In the United States, the paper industry produces over 80 million tons of paper annually. It is estimated that the amount of dry waste, waste sludge with substantially all of the residual water removed, produced due to paper processing exceeds 4.6 million tons per year. Of the 4.6 million tons of dry waste, up to 1.5 million tons is fiber suitable for reuse in the industry. The remainder of the waste includes clay, ash and fiber fines (short fibers unsuitable for paper making) which may potentially be used as filler material in plastics or other material, or for other purposes. The dollar value of this recoverable fiber can range from $200 to $400 per ton or a total value of approximately $300 to $600 million dollars per year. Not included in this dollar amount are the benefits from reduced strain on the environment, i.e., less land fill space occupied by reusable materials, and less virgin wood pulp required for paper making. The benefit and value of recovering the fiber material from this waste sludge is thus readily apparent. However, to be economically feasible, the cost per ton of fiber must be competitive with the cost of producing fiber from virgin wood pulp and alternate recovered fiber sources.
In this regard, there are known "wet" processes of recovering fibers from paper making sludge. One such process, and variations of it, is the subject of U.S. Pat. Nos. 4,983,258; 5,002,633 and 5,137,599 all to Maxham. In Maxham's process, paper making sludge, either dewatered or not, is treated in a slurry with detergents and other defibering agents in a primary stage for releasing fibers from the sludge. If the sludge was previously dewatered, water is added at this primary stage to the sludge to maintain a solids concentration of approximately 20 percent. The slurry is then screened to remove clay and fiber fines and to capture the fibers. The fibers, now in a substantially dewatered suspension, are then communicated to a mixing tank where additional water is added and the fibers are further agitated. This mixture is then communicated to a hydrocyclone which imparts a centrifugal force to the slurry to remove debris having a specific gravity greater than water from the fibers. The slurry now containing substantially debris free fibers is again screened. The fibers are then carried to a bleaching tank and finally to a dewatering device. Associated with the primary processing described, is a substantial amount of secondary processing for collecting fiber fines, clay, filler materials, debris and the like as well as for processing and treating the water used during the entire process such that it may be reused or safely returned to the environment.
Wet processing of paper making sludge, such as described above, suffers a number of disadvantages. For example, the wet processes generally require a significant number of steps, and the introduction of detergents or other chemicals to facilitate separating fibers from the sludge. Also, unless the processing facility is located closely adjacent to the paper making mill, the sludge must first be dewatered for economical transportation to the processing facility, diluted for processing and then dewatered an additional time after processing. The water used in the process must be treated such that it is safe for reuse or for return to the environment. These added primary and secondary processing steps require additional energy and facilities which reduce the overall efficiency of the process and therefor raise the overall processing cost per ton of the fiber produced. Also, the processes have been found to be less efficient with sludge dewatered by particular dewatering equipment such as screw dewatering machines, and in general, wet processes require rather large processing facilities, including holding tanks and basins, pumping equipment and the like.